team: add missing attribute validation for array index

Add missing attribute validation for TEAM_ATTR_OPTION_ARRAY_INDEX
to the netlink policy.

Fixes: b1303326 ("team: introduce array options")
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>